Low-Temperature Roasting of Ilmenite — Phase Chemistry and Applications

Removal of gangue minerals from ilmenite concentrates can be accomplished by relatively low-temperature roasting in controlled gas atmospheres to selectively enhance the magnetic separation characteristics of the ilmenite. At roasting temperatures below 800°C, the magnetising reactions are often not consistent with phase equilibria considerations. Metastable phases can nucleate and control the roasting products. In weathered ilmenites, the phase assemblages and microstructures in the altered ilmenite grains can influence the roasting reactions. This paper gives an overview of the phase chemistry of chemically weathered ilmenites and of metastable phases formed by low-temperature roasting. We report the results of roast experiments conducted in a neutral gas atmosphere. Major increases in magnetic properties after short roasting times at low temperatures were obtained for a range of ilmenite concentrates. For example, heating a secondary ilmenite concentrate from Western Australia for 15 minutes in nitrogen at 600°C gave an increase in the three kilogauss magnetic fraction from three to 92 wt per cent. The solid state phase changes accompanying the inert atmosphere roasts were characterised by combining elemental analyses with quantitative phase analyses of the ilmenite concentrates and their roast products.